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GHD prepared this advisory initially in 2015 to assist our clients that were in the process of developing the groundwater monitoring program required by 40 CFR Parts 257 and 261. We have updated this advisory to include a description of important geochemical forensic tools that can assist in the interpretation of the more recently collected groundwater monitoring data.

Under this new federal rule for Coal Combustion Residuals (CCR) issued by the United States Environmental Protection Agency (USEPA) on April 17, 2015, there are a number of groundwater monitoring requirements associated with CCR management units. The groundwater monitoring requirements outlined in the USEPA CCR Rule include the sampling and monitoring of groundwater for background purposes, detection programs, and assessment situations as well as during corrective action. These monitoring programs are focused on the quality of groundwater coming into an area of CCR units, the quality of the groundwater flowing under and away from these CCR units, and any potential release from a CCR unit. This advisory presents the information that our clients will need when implementing updated monitoring programs for their facilities. In addition, this advisory identifies unique evaluations that can be used to evaluation the groundwater data.

Groundwater Monitoring Requirements

The Chemicals of Concern (COCs) for the above monitoring programs are listed specifically in Appendix III and Appendix IV of the new USEPA CCR Rule. Those COCs generally comprise inorganics and metals. The COCs that must be monitored by the USEPA CCR Rule include naturally occurring inorganics (e.g., chloride and arsenic) and naturally occurring metals (e.g., boron and selenium) that are found commonly in groundwater and in the mineralogy of natural deposits. These inorganic parameters and metals are subject to changes in their solubility due to natural processes such as oxidation or reduction reactions and the activity of indigenous microbes under either aerobic or anaerobic conditions. Therefore, the subject COCs comprise a list of parameters that are commonly found in hydrogeologic environments (regardless of the presence of CCR units). These COCs are also subject to changes in their dissolved concentrations due to natural variations in environmental conditions (regardless of the presence of CCR units). An appropriate groundwater monitoring program, as required by the USEPA CCR Rule, must take into account the fate and transport characteristics of these naturally occurring COCs.

The USEPA CCR Rule allows for the identification of ambient or background conditions (Sections 257.93 [d to j]) for the specific COCs listed in Appendix III and Appendix IV of the Rule to separate out those COCs occurring under natural conditions from potential releases from an inactive or active CCR unit. These baseline conditions are established under the assumption of common and continuous environmental conditions across a site. In other words, the background is established assuming that the following conditions are the same during future monitoring periods:

The background program outlined in the CCR Rule does not require the monitoring of changes in the overall environmental conditions as indicated by the criteria listed above. Without the proper establishment of these background environmental conditions within the groundwater system (flowing under the CCR unit), it will be difficult to discern whether an increase in a COC concentration is related to changes in overall groundwater conditions or whether the increase is due to a release from the CCR unit itself. Therefore, GHD recommends that any baseline or background monitoring of groundwater near the CCR units include the environmental indicators listed above (i.e., DO, Redox, microbes, and select metals). Not only are these indicators important in establishing an accurate detection monitoring system, but these same parameters are critical to the post-closure or post-corrective action phase at any CCR unit.

As is expected, many inactive and active CCR units will be closed because they do not meet the "Location" criteria. These closure actions will generally involve the capping of the current or former impoundment, the excavation and placement of the CCR into a new lined landfill, or the in situ stabilization of the CCR material below the water table. In addition, the corrective action for a CCR unit that is found to be leaking will require similar closure actions (i.e., capping, in situ stabilization, or removal). These closures and corrective actions will involve significant changes in precipitation recharge and changes in groundwater flow directions. These changes will have a direct impact on the overall environmental conditions discussed above (i.e., DO, Redox, microbes, and dissolved metal concentrations)2.

The changes in DO, Redox conditions, and microbial activity that would accompany a new landfill cap or large-scale in situ stabilization project would likewise affect the concentrations of the COCs in groundwater at the edge of that CCR unit or at other nearby locations. Under these circumstances, there might be an increase in specific COCs that are related to the changes in redox conditions and/or increase in anaerobic microbial activity caused, for example, by a new cap on the former impoundment.

Over the last 30 years, GHD has developed a comprehensive understanding of the potential geochemical changes to a groundwater system as result of the implementation of corrective actions or remedial measures. For example, we have been involved with a number of remedial projects where significant increases in arsenic levels downgradient of a former landfill occurred after the cap was installed. It is common for arsenic to mobilize under such anaerobic conditions. There are numerous references and past project histories that document these types of changes in groundwater conditions as a result of landfill closure or other corrective actions. (Kumpiene et al., 2008; Rochette et al., 1998; Hudgins and Harper, 1999; Hildum, 2013; and Visvanathan et al., 2004).

Many of the trace metals and other inorganics associated with CCRs are also subject to significant changes in dissolution or precipitation with changes in Redox, pH, DO, and microbial activity. Recent research (WOCA 2015) has demonstrated that concentrations of CCR-related COCs such as boron, selenium, molybdenum, lead, and arsenic, which are dissolved in groundwater, will change when the surrounding environmental conditions are modified.

In summary--given the sensitivity of the fate and transport of CCR-related COCs in groundwater to DO, Redox, microbial activity, and natural mineralogy--these parameters need to be monitored prior to and during the implementation of CCR unit closure or corrective action. GHD recommends that, at a minimum, the following field parameter data (in addition to those listed in the USEPA CCR Rule) be collected:

DO

Redox (ORP)

pH (monitored through a flow through cell)

The following laboratory analyses are suggested to be added to the Appendix III and IV list to better establish environmental conditions of the subject groundwater system3:

Dissolved and total iron and manganese

Total organic carbon (TOC)

Nitrate

Dissolved Methane

The field parameters listed above can be collected during the sampling event at very little cost during the documentation of other field parameters such a pH, temperature, turbidity, and fluid levels. The analytical costs for the iron and manganese analyses will range from $38 to $72, dependent upon laboratory contracts. The costs for the TOC analyses range from $20 to $40, the costs for nitrate analyses range from $10 to $20, and the dissolved methane analyses will range from $115 to $200 per sample. These additional costs are minimal when compared to the costs of assessments and re-sampling that will be required at a later date when new COCs are detected or existing COCs have increased concentrations. These additional analyses and tests should be performed quarterly for 1 to 2 years, if possible, prior to the implementation of a closure plan or a corrective action. These events will allow quantification of the seasonal variations within the groundwater system. At the minimum, at least one set of these data would be needed prior to the capping, lining, or in situ stabilization at a CCR unit.

Forensic Evaluation of Groundwater Monitoring Data

During the course of a detection monitoring or a post-closure monitoring program, unexpected detections of CCR-related COCs may be present in samples collected from one or more monitoring locations. There are several potential sources of COCs in groundwater that are not due to CCR disposal activities. For example, arsenic can be found in residues from mining and smelting, wood treatment, municipal landfill, pesticides, fertilizers, and is also found naturally is aquifer sediments. Determining the source of these COCs detected in the groundwater monitoring network may save millions of dollars in potential litigation and clean-up costs, if the source of the COCs is not related to the CCR disposal activities.

The following presents some of the geochemical forensic tools that are available and can be used to determine the source of COCs in a groundwater sample.

Specific Chemical Focus

Geochemical forensic tools may include the use of relationships between major elements, rare earth elements and trace elements, the use of elemental ratios (e.g. Si/Al, Ca/Sr, Pb/Al), fallout radionuclides, isotopic ratios, biogeochemical tracers and mineral ages. Furthermore, mineralogy, the presence or absence of heavy minerals, the chemistry of clay minerals, and grain morphology and texture may also assist in determining the source of a specific contaminant. The following section briefly presents the potential alternate sources of select coal-ash COCs and some of the most powerful geochemical tools specific to fingerprinting contaminants from CCR activities that can be used to determine their source(s). These are only some of the tools available to geochemists; the use of the right tool depends on an understanding of site conditions and history.

Arsenic

Arsenic is a naturally-occurring metalloid, often associated with sulfur minerals, and is found in coal and coal ash, in metal smelter tailings and ash, and often occurs adsorbed onto oxide coatings on soil grains. The presence of arsenic in groundwater does not necessarily indicate that CCR-related impacts have occurred. A careful evaluation of the groundwater chemistry can help determine the origin of the arsenic. For example, a high lead/arsenic ratio is sometimes indicative of impacts from a smelter (Morrison and Murphy, 2006). Elevated copper and chromium concentrations can indicate that the arsenic may be from wood preserving activities. Naturally-occurring arsenic can also be mobilized from the soil in response to a decrease in the redox state of the groundwater (USGS, 2007, Cozzarelli et. al., 2016). By examining the redox conditions at a site (as discussed in Section 2.0), as well as the concentrations of related, and unrelated analytes, the source of arsenic in groundwater can be determined.

Boron

Boron condenses in the coal ash generated from coal combustion plants. During the combustion of coal, boron is partitioned into the bottom ash, the flyash retained by particle attenuation, and fine flyash, some of which reaches the atmosphere with the stack gases (Goodarzi and Swaine, 2007; Cox, et al. 1978). As the combustion of coal subsequently contributes boron to the atmosphere, it is important to be able to pinpoint the anthropogenic sources of boron into the environment.

The isotopic signature of boron from coal ash is distinctive from naturally occurring boron, or boron from other anthropogenic sources. Using strontium and boron isotopes together can identify coal ash contaminants within the environment, and can be used to provide definitive evidence to determine whether contamination is related to coal ash or another source (Ruhl, et al., 2014). Most importantly, the use of these isotope tracers can allow coal ash contamination to be distinguished from other, similar contamination coming from different sources in a catchment, while also linking other contaminants associated with coal ash back to the source.

Lithium

Lithium isotopes have been shown to be useful tools in delineating coal related contamination in the environment fluids (Harkness, et al. 2015). Lithium isotopes have a distinctive isotopic signature with respect to coal, particularly when coupled with boron or strontium isotopes. This multi-isotope relationship is an important tool for distinguishing contaminants sourced from multiple sources, particularly those associated with CCR to those from oil and gas wastewater, including conventional produced waters and hydraulic fracturing fluids (Harkness, et al. 2015).

Molybdenum

Molybdenite (MoS2) is used in the petroleum industry as a catalyst for the hydrogenation of coal and to remove sulfur in oil. The different industrial sources of anthropogenic molybdenum contribute to its concentration in the atmosphere. Molybdenum from these sources is ultimately dispersed through the atmosphere, before settling in water bodies and sediments. To discriminate between naturally occurring and anthropogenic molybdenum, isotope values (δ98Mo) may be used as a fingerprinting tool to determine the source of the contaminant (Chappaz, et al. 2012).

Geochemical Forensic Tools

The following section presents some of the geochemical tools that can be used to determine the source of impacts detected during groundwater monitoring. Each situation requires careful evaluation of the site conditions in order to determine the best tool for the job.

FALCON Procedure

The USEPA developed a multi-variant contaminant fingerprinting procedure called the Fingerprint Analysis of Leachate Contaminants (FALCON) procedure (USEPA, 2004). The procedure involves combining the concentrations of several parameters to create a signature (fingerprint) of a contaminant plume or source. These fingerprints can be used to determine the probability that a downgradient well in the monitoring network has been impacted by water released from the site, or by COCs from another source.

The FALCON procedure works by characterizing the ratios between chemical constituents of a source water. If the water from that source is simply diluted, then the ratios will remain the same, regardless of the dilution factor. However, if a separate source is contaminating the groundwater, the ratios of COCs to one another will not be identical, and using the FALCON procedure will allow us to detect this new source.

Like any real fingerprinting, chemical fingerprinting requires a sample of unknown origin (the potentially impacted well) and a reference sample for comparison (leachate from the site, and/or background water). Collection of the appropriate data before problems occur can save time and money.

There are other procedures that rely on the ratio of one element to another (e.g., arsenic and lead) to determine the likely source or relationship between samples. The use and success of these methods relies on an understanding of the each site's history, surroundings, and geochemistry.

Isotopes

Isotopes are atoms of an element that have a different number of neutrons, giving them different masses. Isotopes of an element will have the same chemical properties, but can be used as tracers because the mass differences between isotopes can be measured in the laboratory. When measuring or discussing isotopes of an element within a sample, geochemists refer to the ratio of the heavy isotope to the light isotope. This isotopic ratio is roughly analogous to the concentration of the heavy isotope in the sample.

The ratio of the heavy to light isotope in a substance can vary depending on the origin of the substance, or the physical and chemical processes that the substance has been subjected to.

For coal ash impoundments, isotopic analysis of some of the major constituents can help determine their origin. For example, in areas where the natural sulfate concentration in groundwater is very high, sulfate concentrations in monitoring wells alone cannot be used as an indicator of impact from a coal ash impoundment. However, the isotopic ratio of sulfur in the sulfate can provide evidence of coal ash-derived impacts. Zielinski et. al. (2005) describe a study where the isotopic ratio of sulfur in sulfate collected from groundwater downgradient of a coal ash impoundment was shown to be distinct from the local background sulfur ratio, and similar to the ratio of sulfur found in sulfate from coal ash.

The isotopic ratios of carbon, hydrogen, and oxygen, among others, can be used in a similar way to help determine the source of groundwater contaminants and of the water itself. It should be noted, however, that isotopes should not be used without a strong understanding of the local hydrogeology and geochemistry, and an isotopic evaluation should only be undertaken as part of a broader sampling program.

Plume Stability Analysis

If COC concentrations have been detected in the monitoring well network, it is crucial to understand the behavior of the COCs in order to make the most appropriate decisions for corrective action, if required. An evaluation of the stability of the COC concentrations over time can provide insight into whether the impacts are recent and increasing over time, or are historical and are decreasing over time.

Two of the tools used to determine plume behavior over time are single-well trend tests and site-wide plume stability analyses.

As the name implies, the single-well trend test uses the data over time within a single well to determine the statistical likelihood that COC concentrations are increasing or decreasing. The results from these tests in conjunction with statistical evaluation of the baseline (pre-emplacement) data can be used to determine if COC concentration changes require remedial action, and the size of the area.

A plume stability analysis as presented by Ricker (2008) uses the data from all site wells to calculate plume area, average plume concentration, and plume mass for each sampling event. These values can then be plotted over time to determine the statistical changes in the plume as a whole, which tends to smooth out variability within individual wells, and provide an overall picture of plume dynamics. The location of the center of mass is also plotted for each sampling event providing an understanding of the overall movement of the plume over time, if any. This holistic approach is included in USEPA guidance documents, and limits the possibility that one or two high results will unduly influence the site decision-making.

We would be happy to discuss the above approach to monitoring and the tools available to evaluate these data in more detail as it applies specifically to your facilities and associated CCR units.

1 Also known as Oxidation Reduction Potential or ORP.2 It should be noted that changes in facility operations not related to the CCR management unit could also have a similar impact on environmental conditions within the subject groundwater system. The facility changes could include, for example, the lining of the raw water feed pond or changes in the chemistry of the cooling water, or other process related changes that impact groundwater quality.3 Additional helpful analytes in the establishment of environmental conditions are total aerobic and anaerobic microbial counts, sulfide, and arsenic speciation.

Hudgins, M. and Harper, S. (1999). Operational characteristics of two aerobic landfill systems. Paper Presented at The Seventh International Waste Management and Landfill Symposium in Sardinia, Italy, on October 4, 1999.

United States Environmental Protection Agency (2004). Fingerprint Analysis of Contaminant Data: A Forensic Tool for Evaluating Environmental Contamination. Office of Researd and Development and Office of Solid Waste and Emergency Response. EPA/600/5-04-054.